A 69-year-old man presents with a 6-month history of progressive fatigue, bone pain (particularly in his back), and recurrent infections. Blood tests show: Hb 95 g/L (normocytic), Calcium 2.95 mmol/L (corrected), Creatinine 180 µmol/L (eGFR 35), Total Protein 95 g/L. What investigation is most crucial to confirm the likely diagnosis?